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Challenges of Aircraft Hangar Fire Protection - The Development and Use of a Modern Standard

By Michael E. Aaron, P.E. | Fire Protection Engineering

Introduction

By
their nature, aircraft hangars pose unique challenges for the fire
protection engineer. There are large, open floor areas with tall roof
decks to house high-value aircraft contents. Large quantities of liquid
jet fuel are present, and aircraft maintenance activities offer a
variety of potential ignition sources.

Another
characteristic that differentiates hangars from most other occupancies
are the large aircraft wings and fuselages that create obstructions to
both fire detection and fire suppression. Sometimes, there are large
scaffolds, which create further obstructions.

Naturally, as hangars come in all shapes and sizes, some of these features are not always present. A 6,000 ft2 (560 m2) shelter for small aircraft poses different challenges than a 150,000 ft2 (14,000 m2) maintenance complex for overhauling commercial jets.

The
main fire threat is posed by a fuel spill finding an ignition source,
leading to a challenging fire. A 50 foot (15 m) diameter pool of burning
Jet-A fuel can produce a heat release rate on the order of 300
megawatts. A few hundred gallons (liters) of ignited fuel is enough to
destroy just about any facility that is not properly protected.

Large
hangars call for suppression systems on a scale with which some
engineers may be unfamiliar. Fire detection systems must function over
unusual heights and distances. Sensitivity is needed for fast response,
but this factor must be balanced against protection from nuisance
alarms. There are a number of fire suppression options, most of which
involve fire pumps, foam systems and sprinkler systems with large design
areas. Zoning and distances from equipment rooms to discharge points
can also create design complications.

No
less challenging for the fire protection engineer is the task of
guiding a client or employer, whether that be building owner,
construction contractor, code official, A/E firm, etc., through the
often confusing array of design options and code requirements. The code
requirements are far reaching and have big cost implications. The costs
can be hard to reconcile against the loss history data. Hangar fires are
low-frequency, but high-consequence events, and the codes require a
large amount of protection and redundancy.

NFPA
409, Standard on Aircraft Hangars,1 is the primary document where
adopted by the local jurisdiction. Like all NFPA codes and standards,
NFPA 409 becomes a legal requirement when referenced in an adopting
ordinance by a local governing body. Sometimes, these ordinances include
amendments to certain provisions of the document. Even when not
specifically adopted, there is often a desire to conform to
internationally recognized minimum standards. Where insurance
requirements govern, compliance with FM Global standards may be
important. For U.S. military projects, matters depend greatly on which
branch of service is involved, as there are differences between criteria
from the Air Force, Navy, Army and National Guard. This article focuses
on NFPA 409.

Historical Perspective

In
the 1950s, NFPA began producing what became NFPA 409, taking the place
of the earlier pamphlet from the National Board of Fire Underwriters
(NBFU). Early NFPA hangar fire protection systems for larger hangars
were based on sprinklers. The requirement became deluge-type sprinkler
systems (with open sprinklers), and allowed a choice of plain water or
foam. If foam was chosen, lesser sprinkler densities were allowed.
Protein-based foams and fluoroprotein foams were used, as well as
synthetic foams, which became the AFFFs (aqueous film forming foam) of
today.

Draft stops (curtains) and
floor drainage were important parts of the protection scheme, so the
deluge flows could wash the burning fuel safely off the floor and down
the drains. In the 1950s and 1960s, large deluge sprinkler systems were
the norm. Rows of original deluge risers are often found in hangars
constructed during this era. They employed pilot sprinklers or
dropweight mechanisms to open the valve clappers. The weights were
released by low pressure pneumatic heat detection systems connected to a
network of heat-actuated devices (known as HADs) installed beneath the
roof deck.

The old deluge systems covered sprinkler zones of up to 15,000 ft 2 (1,400 m2) that were separated by draft curtains. They had design densities of 0.16 to 0.25 gpm/ft2
(6.5 - 10 mm/min). The number of simultaneously flowing zones to be
hydraulically calculated was determined by what was known as the "radius
rule." The higher the roof deck, the larger the radius of an imaginary
circle drawn in the plan view. Any zone touched by the circle had to be
included. Hangars with 4, 5 or 6 zones calculated flowing were common,
leading to huge sprinkler design areas of 90,000 ft2 (8,000 m2) or more.

With
the advent of wide-body aircraft with expansive wing areas, such as the
Boeing 747, the NFPA 409 committee became concerned that the aircraft
would shelter a fire from the sprinkler discharge, and the water or foam
would be too slow to reach the fire. They saw the need for foam to be
discharged directly beneath the aircraft. With the 1970 edition, NFPA
409 began requiring "supplemental” foam systems in addition to the
deluge sprinklers where there were individual aircraft with shadow areas
greater than 3,000 ft2 (280 m2).

Supplemental
systems almost always employed oscillating monitor nozzles. (High
expansion foam is also an option.) Though these nozzles need to only
cover the area beneath the aircraft, as a practical matter they must
cover a considerably larger area in order to reach all parts of the
irregular shape of the aircraft shadow.

In
the 1970s and early 1980s, Factory Mutual conducted research that led
to the conclusion that sprinklers discharging plain water would fail to
control a pool of burning jet fuel on a hangar floor.2
Increasing sprinkler densities was not the answer, since fuel rises
above water and can continue to burn or vaporize and reignite.

Because
of their physical properties, foams stay above and cling to the surface
of burning fuels with a smothering action that provides cooling, cuts
off oxygen and suppresses fuel vaporization. Therefore, star ting with
the 1985 edition, NFPA 409 eliminated the option of plain water deluge
sprinklers for Group I hangars, allowing only foam-water deluge
sprinklers.

In the late 1990s, the NFPA 409 committee was presented with research conducted by the U.S. Navy.3
This led the 2001 edition to incorporate the most significant changes
to Group I hangars since the foam requirement. The traditional
foam-water deluge sprinkler scheme was retained, but as just one of
three possible options. The old radius rule governing these deluge
designs was revised to be independent of roof height.

The
two new options were variants of the Group II protection requirements.
In these new options, closed-head sprinklers are used at the roof level,
and foam systems, either low-expansion or high-expansion types, are
employed to cover the entire hangar floor area. These are termed
"low-level” foam systems. Thus, the general historical trend has been to
reduce the role of sprinklers from the primary fire suppression system,
to a system to cool the steel while a foam system blankets the floor.

Understanding and Applying NFPA 409

The
first step in applying NFPA 409 is to address the basics: will the
aircraft in the hangar always be unfueled? What "group” should this
hangar be classified as?

Allowing
only unfueled aircraft in the hangar reduces protection requirements to a
simple hazard sprinkler system. Most owners find this unacceptable for
their operations. Fueled aircraft are the norm. Regarding hangar groups,
rules-of-thumb (full details are in the standard) are as follows:

If the aircraft bay is greater than 40,000 ft2 (3,700 m2) and/or if the hangar door is taller than 28 feet (8.5 m), it’s a Group I (the most severe case).

If neither condition is true, it’s a
Group II (only somewhat less severe, still lots of requirements,
including foam and sprinklers).

Unless it’s a lot smaller (12,000 ft2 [1,100 m2]
or less for common construction types), in which case it’s a Group III.
(Few requirements: no sprinklers or foam, no fixed fire suppression
systems at all, as long as there are no hazardous activities such as
welding, painting, etc.)

Finally if the hangar is a "membrane covered rigid steel frame”1
and larger than a Group III, then it’s a Group IV. (A foam system or
closed-head sprinkler system is required.) This relatively new type of
hangar construction is becoming more popular.

An
owner may be interested in considering construction options that allow
the facility to be classified at a lower protection level. In some
cases, there may be compromises that afford substantial cost savings.
Therefore, it’s useful to know where the lines are drawn.

General
requirements for construction and passive fire protection for both
Group I and Group II hangars are found in Chapter 5. An abbreviated
summary of the main requirements are:

Construction must be non-combustible.

Egress must meet NFPA 101, The Life Safety Code.

For hangar fire areas to be considered
(calculated) separately, 3-hour walls are needed between aircraft bays.
Otherwise, multiple bays are considered as one area with larger water
and foam demands.

Shops and office areas must be separated from the aircraft bay by 1-hour rated walls.

Building columns in aircraft bays must
have 2-hour protection or the columns must be sprinklered.

Hangar door power must be connected
ahead of the building disconnect and must be operable in an emergency.

Trench drains are required and must have
the capacity to carry away the full design fire flow rate of the fire
suppression systems.

Any pits or tunnels in the hangar floors must be
mechanically ventilated, drained, and treated as Class 1, Division 1
hazardous areas per the National Electrical Code.

For Group I hangars only: draft curtains
must be provided, enclosing projected floor areas of 7,500 ft2 (700 m2)
or less. These draft curtains do not define sprinkler zones and are not
needed in Group II hangars.

For Group I hangars, fire suppression
requirements are found in Chapter 6 and in Chapter 7 for Group II
hangars. The main differences between Group I and Group II hangars are:

When closed-head sprinkler systems are chosen, the Group I design criteria is 0.17 gpm/ft2 (7 mm/min) over 15,000 ft2 (1,400 m2), while Group II systems use the same density but with only a 5,000 ft2 (460 m2) design area.

Water flow duration times are
approximately 50% longer for Group I hangar systems than for those of
Group II.

Draft curtains are not required in Group II hangars.

Group I Choices

Since
the options with closed-head sprinklers plus low-level foam systems
became available, foam water deluge sprinklers are seen less often. This
is particularly true when there are large aircraft with wing areas of
more than 3,000 ft2 (280 m2), which invokes the
need to add supplemental foam systems. It’s usually more economical to
provide a larger low-level foam system instead of a supplemental foam
system because the deluge system can be eliminated in favor of closed
head sprinklers with a design area of 15,000 ft2 (1,400 m2). Going through the exercise of estimating these demands bears this out.

High-expansion
foam usually leads to lower water demands than with other options,
sometimes making it an attractive choice. If high-expansion foam is
selected as a low-level system, NFPA 409 calls for the foam generators
to utilize outside air. This means that the foam generators need to be
ducted through the roof to intake hoods. Louvers and dampers will also
be needed for relief air. U.S. Air Force and Army design criteria allow
the use of inside air, which simplifies matters considerably. Some AHJs
may be willing to consider the military approach of using inside air.

Common Sources of Confusion

Low-level
vs. supplemental systems is perhaps the single greatest area of
confusion in the standard. Supplemental systems are only provided in
conjunction with foam water deluge sprinkler systems. Supplemental
systems need to cover only the area beneath the aircraft, while
low-level systems must be calculated for the entire hangar floor area.
The design, objectives and testing requirements for each are different.

High-expansion
foam is often used as a low-level system, although the foam generators
are usually installed up high, not close to the floor. Low level systems
are so named because their purpose is to cover the floor.

NFPA
409 provides a method for calculating the application rate of
high-expansion foam. This method does not call for maintaining a
submergence volume, because this is a local application approach, rather
than a total flooding approach. The high-expansion foam is intended to
act in a 2-dimensional manner. Therefore, it does not matter if the
hangar doors are open or closed during foam discharge.

While
the capacities of the water supply and foam system must be designed for
complete coverage of the hangar floor, that does not mean they must all
be activated simultaneously. The systems can and should be zoned in
order to be discharged in response to heat detection on a zone-by-zone
basis. Should the maximum number of zones be needed, the capacity must
be available.

Redundancy is specified in different ways for water storage, for fire pumps and for foam supply.

Water Storage:
Should stored water be necessary, a divided supply is required. The
idea is for half the water to be available if one tank is down for
repair. It does not mean the water storage quantity is to be doubled. If
200,000 gallons (900 m3) are needed to provide 45 minutes of f low
duration, a pair of 100,000 gallon (450 m3) tanks should be provided.
They should be piped to be used at the same time.

Fire Pumps:
Should fire pumps be necessary (as they frequently are), the number and
size of the fire pumps needed should be determined, and one additional
pump of the same capacity should be provided. The requirement is for
full pumping capacity with one pump out of service. Redundant jockey
pumps are not required. Suction pipe sizing need not consider the
redundant fire pump.

Foam Concentrate Pumps:
If foam pumps are being used, they are treated in the same manner as
fire pumps as far as redundancy is concerned. The schematic piping
diagrams in the annex of NFPA 11 do not show all required components,
and they do not show how multiple pumps are to be connected.

Foam Tanks:
The requirement is for a "connected reserve” tank. The primary tank is
to have full capacity for the 10-minute duration in the case of low
expansion foam, or 12 minutes in the case of high-expansion foam. The
connected reserve tank is to be of the same size as the primary tank.
Its purpose is to be readily available after an event so the system may
be put back into service quickly. As such, it should be connected so
that manual operation of valves is needed to utilize its contents. This
is true for both pressurized diaphragm tanks and atmospheric pressure
tanks.

NFPA
409 is a prescriptive standard. If one wants to vary from its methods,
NFPA 409, like most standards, allows for equivalencies or new
technologies as long as the level of protection is not lowered. The
authorities having jurisdiction have considerable discretion in this
area if they choose to exercise it. Alternatives may be considered if
one can provide analysis with enough rigor to satisfy the AHJ that a
proposed alternative provides an equivalent level of protection.

About SFPE

SFPE is a global organization representing those practicing in the fields of fire protection engineering and fire safety engineering. SFPE’s mission is to define, develop, and advance the use of engineering best practices; expand the scientific and technical knowledge base; and educate the global fire safety community, in order to reduce fire risk. SFPE members include fire protection engineers, fire safety engineers, fire engineers, and allied professionals, all of whom are working towards the common goal of engineering a fire safe world.